flutter – Firebase, how to implement scheduler?

When some information is stored in the firestore, each document is storing some specific time in the future, and according to that time, the event should occur in the user’s app.

The first way I could find was the Cloud Function pub sub scheduler. However, I could not use this because the time is fixed.

The second method was to use Cloud Function + Cloud Task. I have referenced this. https://medium.com/firebase-developers/how-to-schedule-a-cloud-function-to-run-in-the-future-in-order-to-build-a-firestore-document-ttl-754f9bf3214a

This perfectly performed the function I really wanted, but there was a fatal drawback in the Cloud Task, because I could only save the event within 30 days. In other words, future time exceeding 30 days did not apply to this.

I want this event to be saved over the long term. And I want it to be somewhat smooth for large traffic.
I`m using Flutter/Firebase, how to implement this requirements above?
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